The present invention relates to radio communications in general, and more specifically, to a method of, and apparatus for, providing beam reduction for a signal combining base station in a radio communication system.
In a digital cellular radio communication system, radio signals which are digitally modulated are used to convey information between radio base stations and mobile stations. The radio base stations transmit downlink signals to the mobile stations and receive uplink signals transmitted by the mobile stations. A common problem that occurs in digital cellular radio communication systems is the loss of information in the uplink and downlink signals as a result of multipath fading and interference which may exist in the radio transmission channel.
With regard to multipath fading, there are basically two effects: fading and time dispersion. When the path length between a mobile station and a base station is relatively short, fading arises from the interaction of the transmitted signal, or main ray, and reflections thereof, or echoes, which arrive at the receiver at approximately the same time. When this occurs, the main ray and echoes add either destructively or constructively. If there are a large number of echoes, the pattern of destructive and constructive addition takes on a Rayleigh distribution, which is why this effect is sometimes called xe2x80x9cRayleigh fadingxe2x80x9d. Where destructive addition results in fading xe2x80x9cdipsxe2x80x9d, the received signal generally exhibits a relatively low carrier-to-noise (C/N) characteristic.
The effects of fading dips can be mitigated by having multiple receive antennas and by employing some form of diversity combining, such as selective combining, equal gain combining, or maximal ratio combining, wherein signals from each receive antenna are combined to create a single received signal. Diversity techniques take advantage of the fact that the fading on the different antennas is not the same, so that when one antenna receives a fading dip, chances are, another antenna is not. Note Mobile Communications Design Fundamentals by William C. Y. Lee, Howard W. Sams and Co., Ind., USA. In section 3.5.1 of this book, several examples are given describing how signals from two receiver amplifiers with separate antennas can be combined to counteract fading.
For longer path lengths, time dispersion occurs when the echoes are delayed with respect to the main ray. If an echo of sufficient magnitude arrives at the receiver delayed from the main ray by an amount of time on the order of the symbol period, time dispersion gives rise to intersymbol interference (ISI). Time dispersion may be advantageously corrected by using an equalizer. In the case of digital signal modulation, a maximum likelihood sequence estimation (MLSE) equalizer such as described in Digital Communications, 2nd Ed., by John G. Proakis, Mc-Graw Hill Book Company, New York, N.Y. USA, 1989 may be used. In section 6.7 of this book, various methods are described for detecting signals corrupted by time dispersion, or inter-symbol interference (ISI), using MLSE equalization.
There may also exist signal sources in the radio environment which are not orthogonal to the desired signal. Non-orthogonal signals, or interference, often come from radios operating on the same frequency (i.e., co-channel interference) or from radios operating on neighboring frequency bands (i.e., adjacent-channel interference). Of course, non-orthogonal signals can have a detrimental affect on the carrier-to-interference ration (C/I) of another channel. When the carrier-to-interference ratio (C/I) of a channel is too low, the quality of voice output at the mobile station is poor. Many techniques have been developed in order to minimize interference to tolerable levels including frequency re-use patterns, frequency hopping and adaptive beamforming, wherein the latter is generally used to steer a null in an antenna pattern in the direction of an interferer.
More recently, methods have been proposed that partially solve the problems of multipath fading and interference. In U.S. Pat. No. 5,191,598 to Bxc3xa4ckstrxc3x6m, et al., for example, the problem of accurately detecting signals in the presence of fading and time dispersion is overcome by using a Viterbi-algorithm having a transmission function estimated for each antenna. By reference thereto, U.S. Pat. 5,191,598 is incorporated herein in its entirety. Another method of accurately detecting signals in the presence of fading and interference was presented in the IEEE Transactions on Vehicular Technology, Vol. 42, No. 4, November 1993, J. H. Winters: xe2x80x9cSignal Acquisition and Tracking with Adaptive Arrays in the Digital Mobile Radio System IS-54 with Flat Fadingxe2x80x9d.
Although the above described conventional techniques can be used to improve signal quality, there remains room for improvement. Thus, in one of the related applications, interference rejection combining (IRC) techniques are described which combat interference, for example, using impairment correlations to improve the maximum likelihood sequence estimation. However, the use of impairment correlations in this way may be mathematically complex and may require significant processor resources. Accordingly, it would be desirable to reduce the calculation complexity while preserving the useful qualities of interference rejection combining.
In accordance with one aspect of the present invention, beamforming and signal combining techniques can be used in conjunction with IRC techniques to reduce the overall complexity of processing received signals. According to one exemplary embodiment of the present invention, signals transmitted from mobile stations are received by an array of antenna elements at a base station. The received signals are combined in a beamforming device, for example a Butler matrix. The outputs of the beamforming device correspond to a plurality of beams which cover an entire cell supported by the base station. The number of beams formed can be smaller than the number of antenna elements, which reduces the complexity of processing within the receiver. Each output of the beamforming device is connected to a radio receiver which converts the RF signal to baseband. Other quality improvement processing functions, including locating the training sequence, estimating the channel impulse response and other quality measures, for example measuring signal strength, noise power and carrier to interference ratio, are then performed. Thereafter, a certain number of received signal branches are selected based upon the estimated quality parameters. This further reduces complexity at the receiver, which performs interference rejection combining on the selected branches.
According to another exemplary embodiment, two or more antenna arrays can be employed to obtain further diversity against fading radio signals. Diversity techniques including, for example, spatial diversity or polarization diversity, can be employed. Like the first described exemplary embodiment, beamforming is also employed to further reduce complexity.